The ability to independently control the braking force at each of the vehicle's wheels enables operation of a vehicle brake system in various special modes of operation. One of these special modes of operation is traction control. During vehicle acceleration, a vehicle wheel may lose traction, and begin to spin. In the traction control mode of braking, the brake system is electronically actuated, without the driver stepping on the vehicle brake pedal, to individually brake the spinning wheel. When the wheel has slowed sufficiently to regain traction, the brake is released.
Another of these special modes of operation is vehicle stability control. Vehicle stability control systems use sensor inputs to calculate the driver's desired path and the vehicle's actual path. If there is a difference between the desired and actual paths, the brake system can be operated to correct vehicle direction by automatically applying/releasing individual wheel brakes.
In a typical braking system having an integrated traction control system and vehicle stability control capabilities, several braking circuits are provided to control the individual wheel brakes of the vehicle. Generally, two braking circuits are provided, with each circuit controlling the operation of two wheel brakes of the vehicle. While the configuration of the individual circuits may vary, each braking circuit typically includes isolation and supply solenoid valves that are used to either restrict or facilitate fluid flow from a shared master cylinder through an individual brake solenoid valve to apply fluid pressure to actuate each individual wheel brake within the braking circuit. A dump solenoid valve is generally also provided in conjunction with each individual brake solenoid valve to release the pressurized fluid from the individual wheel brakes and to allow the pressurized fluid to flow to an accumulator. A pressurized fluid source, such as a pump or high pressure accumulator, is generally located in each braking circuit to pump the fluid back through the system. Therefore, multiple pressurized fluid sources are used within the braking system, as a pressurized fluid source is provided within each individual braking circuit to control the fluid flow therein. The individual braking circuits may also include other fluid flow devices, such as attenuators, check valves, and/or restricted orifices, depending upon the system design.
It is known from the prior art to cross-connect the individual braking circuits to allow certain individual components present in each of the braking circuits to work in conjunction with the companion component in the other braking system. For example, U.S. Pat. No. 5,567,022 to Linkner, Jr. discloses an anti-lock braking system that cross connects the individual braking circuits using an elastomeric element to attenuate the pressure pulse energy dissipation provided by each of the attenuators in the individual braking circuits. The cross-connection of the braking circuits via the elastomeric element allows the attenuator action from one individual braking circuit to complement the attenuator action of the other individual braking circuit, which doubles the attenuator effectiveness and reduces noise within the anti-lock braking system. It is also known from the prior art to provide connection devices within a braking system to maintain fluid pressure balances within the braking system. For example, U.S. Pat. No. 5,890,364 to Linkner, Jr. et al. discloses a compensation piston arrangement that balances the fluid pressure in a dual chamber master cylinder that is divided such that each of the individual fluid chambers within the master cylinder provides fluid to an individual braking circuit within a vehicle braking system. The compensation piston operates to maintain generally equal fluid pressures within the master cylinder fluid chambers to compensate for minor normal differences between brake circuit fluid displacement.
Where each of the braking circuits are connected to a single chambered master cylinder, fluid is provided to each of the individual braking circuits from the master cylinder whenever operation of any of the individual wheel brakes in either braking circuit is required. This causes fluid to flow through each of the braking circuits, which in turn causes the pressurized fluid source in each of the braking circuits to operate to supply fluid pressure within the circuit. During certain braking operations, such as traction control or vehicle stability control operations, the control mechanisms of each braking circuit, i.e. the supply and isolation valves, allow fluid to flow to only one of the individual wheel brakes, which results generally in the utilization of only one of the individual braking circuits. Therefore, the output of the pressurized fluid source in the non-utilized braking circuit is not used, meaning the output of the pressurized fluid source does not supply fluid pressure to the wheel brakes within the braking circuit, and the pressurized fluid source operates only to supply fluid back to the master cylinder fluid reservoir.